This invention relates generally to methods for beneficiation of kaolin and other clays, and more specifically relates to a method for improving the brightness of kaolin clays or the like through the conjunctive use of a high intensity magnetic field.
Natural clays, including kaolin clays, frequently include discoloring contaminants in the form of iron and/or titanium-based impurities. The quantities of titanium-based impurities are particularly significant in the case of the sedimentary kaolins of Georgia, where such impurities are commonly present as iron-stained anatase and rutile. In order, therefore, to refine the clay and bring the brightness characteristics of the resultant product to a level acceptable for paper coating applications, various techniques have been used in the past to remove such discoloring impurities. Thus, for example, hydrosulfites have been widely used for converting at least part of the iron-based (or "ferruginous") impurities to soluble forms, which may then be removed from the clay.
Among the most effective methods for removing titaniferous impurities, including e.g. iron-stained anatase, are the well-known froth flotation techniques. According to such methods an aqueous suspension or slurry of the clay is formed, the pH of the slurry is raised to an alkaline value, for example, by the addition of ammonium hydroxide, and a collecting agent is added, for example, oleic acid. The slurry is then conditioned, by agitating same for a relatively sustained period. A frothing agent, for example, pine oil, is added to the conditioned slurry, after which air is passed through the slurry in a froth flotation cell, to effect separation of the impurities.
Within recent years it has been contemplated that magnetic separation techniques might be utilized in order to remove certain of the aforementioned impurities, including titaniferous impurities as well as certain ferruginous matter. Anatase, for example, and certain other paramagnetic minerals have been found to respond to high intensity magnetic fields. Thus in U.S. Pat. No. 3,676,337 to Henry H. Kolm, for example, a process is disclosed for treating slurries or the like by passing same through a steel wool matrix in the presence of a background field of at least 12,000 gauss. Such process has been found useful in removing the aforementioned contaminants from kaolin slurries. The apparatus disclosed in Marston, U.S. Pat. No. 3,627,678, is similarly utilizable in separating the aforementioned impurities from a clay slurry. In this latter instance, the slurry to be treated is thus passed through a canister including a stainless steel or similar filamentary matrix, while a high intensity magnetic field is impressed on the said matrix.
A study of the prior art including the patents cited, will demonstrate that prior researchers have considered flotation technology on the one hand and magnetic separation treatment on the other, to be alternate approaches to the impurity-removal problem. The explanation of this appears to be that froth flotation has been considered so effective in removing titaniferous impurities, that it has been believed that no significant advantage could derive from subsequent use of magnetic separation -- except for removal of ferruginous matter, which in any event is removable by normal leaching.